Deposition method and apparatus

ABSTRACT

A method of depositing a material on a substrate comprises placing a substrate into a process space in fluidic communication with a Gaede pump stage (GPS). A precursor gas is then injected into the process space while injecting a draw gas at a draw gas flow rate into the GPS such that the injected precursor gas achieves a precursor pressure and a precursor gas flow rate in the process space. Subsequently, substantially all of the precursor gas remaining in the process space is swept from the process space by injecting a sweep gas into the process space such that the injected sweep gas achieves a sweep pressure and sweep gas flow rate in the process space. The precursor pressure is higher than the sweep pressure, and the precursor gas flow rate is lower than the sweep gas flow rate.

FIELD OF THE INVENTION

The present invention relates generally to methods and apparatus fordepositing materials on substrates, and, more particularly, to methodsand apparatus for depositing materials on substrates using atomic layerdeposition with modulated pressures and gas flow rates.

BACKGROUND OF THE INVENTION

Atomic layer deposition (ALD) provides highly conformal materialcoatings with exceptional quality, atomic layer control, and uniformity.Coatings deposited by ALD are, for example, well suited for protectingmany products from corrosion and harsh ambient conditions. Effectivecorrosion protective ALD coatings may only be about 200 to about 1,000nanometers (nm) thick, making them thin enough not to impact thedimensions or the bulk properties of most of the parts and products onwhich they are deposited. Moreover, ALD coatings typically displayexcellent conformality and hermetic sealing properties. As a result,potential applications for ALD coatings are wide ranging. They includemicroelectronic packaging, medical devices, microelectromechanicalsystems, carbon nanotube assemblies, flat panel displays, high-endconsumer and aerospace parts, printed circuit boards, tools, solarpanels, and a myriad of other applications.

Fundamentally, repetitive ALD process cycles consist at the very minimumof two reaction sub-steps. Typically, in a first reaction sub-step, asubstrate is exposed to a first precursor gas ML_(x) having a metalelement M (e.g., M=Al, W, Ta, or Si.) that is bonded to an atomic ormolecular ligand L. The substrate surface is typically prepared toinclude hydrogen-containing ligands AH (e.g., A=O, N, or S). Thesehydrogen-containing ligands react with the first precursor gas todeposit a layer of metal by the reaction:

substrate-AH+ML_(x)→substrate-AML_(x-1)+HL  (1)

where the hydrogen containing molecule HL is a reaction by-product.During the reaction, the AH surface ligands are consumed, and thesurface becomes covered with L ligands from the first precursor gas,which cannot react further with that gas. As a result, the reactionself-terminates when substantially all the AH ligands on the surface arereplaced with AML_(x-1) species. This reaction sub-step is typicallyfollowed by an inert-gas (e.g., N₂ or Ar) sweep sub-step that acts tosweep substantially all of the remaining first precursor gas from theprocess space in preparation for the introduction of a second precursorgas.

The second precursor gas is used to restore the surface reactivity ofthe substrate towards the first precursor gas. This is done, forexample, by removing the L ligands on the substrate and re-depositing AHligands. In this case, the second precursor gas typically consists ofAH_(y) (e.g., AH_(y)=H₂O, NH₃, or H₂S). The reaction:

substrate-ML+AH_(y)→substrate-M-AH+HL  (2)

converts the surface of the substrate back to being AH-covered (notethat this reaction as stated is not balanced for simplicity). Thedesired additional element A is incorporated into the film and theundesired ligands L are substantially eliminated as volatile by-product.Once again, the reaction consumes the reactive sites (this time, theL-terminated sites) and self-terminates when those sites are entirelydepleted. The remaining second precursor gas is then removed from theprocess space by another sweep sub-step.

The sub-steps consisting of reacting the substrate with the firstprecursor gas until saturation and then restoring the substrate to areactive condition with the second precursor gas form the key elementsin an ALD process cycle. These sub-steps imply that films can be layereddown in equal, metered cycles that are all identical in chemicalkinetics, deposition per cycle, composition, and thickness. Moreover,self-saturating surface reactions make ALD insensitive to precursortransport non-uniformities (i.e., spatial non-uniformity in the ratethat the precursor gases impinge on the substrate) that often plagueother deposition techniques like chemical vapor deposition and physicalvapor deposition. Transport non-uniformities may result from equipmentdeficiencies or may be driven by substrate topography. Nonetheless, inthe case of self-saturating ALD reactions, if each of the reactionsub-steps is allowed to self-saturate across the entire substratesurface, transport non-uniformities become irrelevant to film growthrate.

As described generally above, an ALD process cycle requires two reactionsub-steps and their associated sweep sub-steps. If each reactionsub-step is further particularized into an injection sub-step, whereinthe respective precursor gas is injected into the reaction space, and areaction sub-step, then a single process cycle actually consists of sixsub-steps in total:

-   -   1. ML_(x) injection    -   2. ML_(x) reaction    -   3. ML_(x) sweep    -   4. AH_(y) injection    -   5. AH_(y) reaction    -   6. AH_(y) sweep.        The highest productivity is achieved when each of these        sub-steps completes as quickly as possible. In fact, because it        frequently requires about 2,000 ALD process cycles to complete        an encapsulation process, each cycle will preferably require        less than about one second. Productivity is, of course, also        affected by other factors. In addition to cycle time,        productivity is also affected by equipment uptime (i.e., the        fraction of the time that the equipment is up and running        properly), cost of consumables (e.g., precursor gases, sweep        gases), cost of maintenance, power, overhead (e.g., floor        space), and labor.

Reaction rates during the reaction sub-steps tend to scale with the fluxof precursor gases on the substrate, which, in turn, scale with thepartial pressure of that precursor gas in the process space. Most ALDprocesses are performed at the low to moderate substrate temperaturerange of about 100-300 degrees Celsius (° C.). At these lowertemperatures, reaction rates are relatively slow or only moderate inspeed. As a result, substantial exposures (e.g., about 10²-10⁵ Langmuirs(L)) of precursor gas may be needed to reach saturation. In these cases,high precursor gas pressure is typically the only way to speed up thereaction sub-steps. Accordingly, reaction sub-steps are preferablyexecuted at the highest possible pressure of undiluted precursor gas. Incontrast, typically very minimal gas flow is needed during the reactionsub-steps to supplement for reactive precursor depletion. Instead,higher gas flow rates will only result in extensive precursor waste.Since many of the precursor gases used in ALD are extremely reactive,un-reacted precursor gas that is swept through the process space swiftlydrives the equipment to malfunction or to fail. It is thereforepreferably that reaction sub-steps are performed with the highestpressures and the lowest gas flow rates.

Effective sweep sub-steps, in contrast, preferably utilize high gas flowrates of the sweep gas to substantially remove any precursor gas fromthe process space before introducing the complementary precursor gasinto this space. Dilution by a factor of about 100-500 during a sweepsub-step is generally considered by those who are skilled in the art tobe sufficient to promote high quality ALD growth. Required sweepsub-step times scale with the sweep residence time, τ_(s)=V×P_(s)/Q_(s),where V is the volume of the process space, P_(s) is the pressure ofsweep gas in the process space, and Q_(s) is the gas flow rate of thesweep gas in the process space. Based on the 100-500 dilution criteria,effective sweep times will exceed about 4.5τ_(s). Based on this formula,one will recognize that, to reduce required sweep sub-step time, processspace volume is preferably minimized when designing the depositionsystem. Moreover, sweep sub-step time may be reduced by using lowersweep gas pressures and higher sweep gas flow rates. The sweep sub-stepstherefore display trends with respect to pressure and gas flow rate thatare opposite to those described above for the reaction sub-steps.

Injection sub-steps drive a concurrent flow-out (“draw”) of sweep gasform the process space while it is loaded with the appropriate precursorgas. The time required for the injection sub-steps scales with injectionresidence time τ_(i)=VP_(i)/Q_(i), where P_(i) is the pressure of theprecursor gas in the process space, and Q_(i) is the gas flow rate ofthe precursor gas in the process space. Accordingly low pressures andhigh gas flow rates allow the injection sub-steps to be faster. Bearingin mind, however, that precursor waste and related equipment failure,downtime, and maintenance are perhaps the most dominant cost factors,best ALD practices generally dictate that injection sub-steps not becarried out beyond 35% volume exchange (e.g., about τ_(i)) under thesegas flow rate conditions. Otherwise, high concentration loading willresult in excessive precursor waste during the injection sub-step. Forexample, to reach greater than 99% concentration of precursor gas in theprocess space during an injection sub-step, the required injection timeof about 4.5τ_(i) will result in more than 58% precursor waste just forthat injection sub-step. This restriction further emphasizes the needfor high pressure during the reaction sub-steps to compensate for lessthan 100% concentrations of precursor gas in the process space.

Based on these trends, one can see that conventional ALD clearly suffersfrom a fundamental tradeoff: injection and sweep sub-steps are madefaster with lower pressures and higher gas flow rates while reactionsub-steps are made faster and less wasteful of precursor gases withhigher pressures and lower gas flow rates. To overcome this tradeoff,process pressure and gas flow rates are preferably modulated in asynchronized manner with the different ALD sub-steps. Nevertheless,driving higher gas flow rates in many apparatus known in the art resultsin higher pressures so that any advantageous effects for ALDapplications are lost. For example, the residence time τ=V×P/Q does notmodulate when both pressure, P, and gas flow rate, Q, are modulated inphase with each other by roughly the same factor. Moreover,pressure/gas-flow-rate modulation techniques known in the art tend toemploy relatively slow mechanical devices that modulate conductance(e.g., throttle valves) or devices that modulate pumping speed (e.g.,devices that change the speed at which a component of the pump moves orrotates). These devices are not practical for the sub-second executionof ALD. For efficient ALD, the time required to modulate pressure andgas flow rates should not ideally exceed 10% of the process cycle time.For example, 100 milliseconds (ms) out of a one second cycle time leavesonly about 25 ms for each pressure/gas-flow-rate transition (there arefour such transitions per process cycle). Excluding other drawbacks, atransition time of about 25 ms is at least 100 times faster than thespeed of most mechanical and pump speed modulation methodologies.

A novel ALD apparatus and method were taught by the inventor of thepresent invention in U.S. Pat. No. 6,911,092, entitled “ALD Apparatusand Method,” commonly assigned herewith and incorporated by referenceherein. Aspects of this invention are shown in the schematic diagramshown in FIG. 1. As indicated in the figure, a “Synchronously ModulatedFlow Draw” (SMFD) ALD system 100 comprises a first precursor gas source101, a sweep gas source 102, and a second precursor gas source 103.These sources are plumbed into a first precursor gas valve 105, a sweepgas valve 106, and a second precursor gas valve 107, respectively, whichcontrol the flow of these process gases into inlets of a process space110. Further downstream, a process space flow restriction element (FRE)115 is attached to an outlet of the process space and carries gas drawnout of the process space into a small-volume draw gas introductionchamber (DGIC) 116. A draw gas source 120 is connected to the DGICthrough a draw gas valve 121 and a draw gas FRE 122. Any gases drawn outof the DGIC enter a DGIC FRE 130 and then an abatement space 132, whichcontains an abatement surface 134. The abatement space is connected toan abatement gas source 138 and an abatement gas valve 139. The systemis pumped by a vacuum pump 140.

The SMFD ALD system 100 is adapted to run process cycles comprising thesix sub-steps described above. During sweep sub-steps, the draw gasvalve 121 is closed and no draw gas is allowed to enter the DGIC 116.This, in turn, allows sweep gases injected into the process space toachieve relatively low pressures and relatively high gas flow rates. Incontrast, during injection and reactions sub-steps, the draw gas valveis opened and draw gas is injected into the DGIC, allowing precursorgases injected into the process space to rapidly achieve relatively highpressures while accommodating relatively low gas flow rates. Moreparticularly, given the small volume of DGIC and the high flow of thedraw gas, a substantial pressure gradient quickly develops over the DGICFRE 130 when draw gas is injected into the DGIC. As a result, pressurein the DGIC quickly increases and the pressure gradient over the processspace FRE 115, ΔP_(Draw), quickly decreases. In this manner, the gasflow rate out of the process space is modulated by effectivelymodulating ΔP_(Draw). If the DGIC has a small volume, very fasttransition speeds may be obtained. For example, a DGIC having a volumeof about 75 cubic centimeters (cm³) implemented within a commerciallyavailable SMFD ALD system designed to deposit materials on eight inchwafer-sized substrates is capable of less than 5 ms transition times.

For gas abatement purposes, an abatement gas from the abatement source138 is introduced through the abatement gas valve 139 into the abatementspace 132 during reaction and the initial stages of sweep sub-steps todrive an efficient reaction with any precursor gases that may havepassed through the process space 110 without being reacted. The productsof this abatement reaction deposit as a solid film on the abatementsurface 134, thereby effectively scrubbing the leftover precursor gaswaste from the exhaust effluent. Advantageously, the high gas flow ratethrough the DGIC 116 during reaction sub-steps effectively separates theabatement space from the process space to allow flexible abatement gasselection without affecting the actual ALD process. Abatementaccomplished in this manner has been shown to extend pump lifesignificantly over that normally seen in conventional ALD systems.

Based on this brief description as well as the details provided in U.S.Pat. No. 6,911,092, it will be clear to one skilled in the art that SMFDALD methods and apparatus provide several advantages with respect toproductivity, efficiency, and cost over other ALD methods and apparatusknown in the art. For this reason, it is desirable to further developnew methods and apparatus for implementation of SMFD-like ALD which mayprovide even greater advantages and capabilities.

SUMMARY OF THE INVENTION

Embodiments of the present invention address the above-identified needby providing methods and apparatus for depositing materials onsubstrates by ALD utilizing Gaede pump stages (GPSs) to modulate thepressures and gas flow rates of precursor gases and sweep gases.

In accordance with an aspect of the invention, a method of depositing amaterial on a substrate comprises placing a substrate into a processspace in fluidic communication with a GPS. A precursor gas is theninjected into the process space while injecting a draw gas at a draw gasflow rate into the GPS such that the injected precursor gas achieves aprecursor pressure and a precursor gas flow rate in the process space.Subsequently, substantially all of the precursor gas remaining in theprocess space is swept from the process space by injecting a sweep gasinto the process space such that the injected sweep gas achieves a sweeppressure and sweep gas flow rate in the process space. The precursorpressure is higher than the sweep pressure, and the precursor gas flowrate is lower than the sweep gas flow rate.

In accordance with another aspect of the invention, an apparatus fordepositing a material on a substrate comprises a process space, a GPS influidic communication with the process space, and one or more gasmanifolds. The process space is adapted to hold the substrate. The oneor more gas manifolds are adapted to inject a precursor gas into theprocess space while injecting a draw gas at a draw gas flow rate intothe GPS such that the injected precursor gas achieves a precursorpressure and a precursor gas flow rate in the process space, and tosweep substantially all of the precursor gas remaining in the processspace from the process space by injecting a sweep gas into the processspace such that the injected sweep gas achieves a sweep pressure andsweep gas flow rate in the process space. The precursor pressure ishigher than the sweep pressure, and the precursor gas flow rate is lowerthan the sweep gas flow rate.

In accordance with even another aspect of the invention, a product ofmanufacture is formed at least in part by placing at least a portion ofthe product of manufacture into a process space in fluidic communicationwith a GPS. A precursor gas is then injected into the process spacewhile injecting a draw gas at a draw gas flow rate into the GPS suchthat the injected precursor gas achieves a precursor pressure and aprecursor gas flow rate in the process space. Subsequently,substantially all of the precursor gas remaining in the process space isswept from the process space by injecting a sweep gas into the processspace such that the injected sweep gas achieves a sweep pressure andsweep gas flow rate in the process space. The precursor pressure issubstantially higher than the sweep pressure, and the precursor gas flowrate is substantially lower than the sweep gas flow rate.

In accordance with an additional aspect of the invention, a GPScomprises an impeller, an enclosure that laterally surrounds theimpeller, a plurality of permanent magnets, and a plurality ofelectromagnetic coils. The impeller comprises a plurality of blades thatspan radially from an inner hub to an outer rim. Moreover, the pluralityof permanent magnets is attached to the outer rim of impeller, while theplurality of electromagnetic coils is attached to the enclosure anddisposed proximate to the outer rim. These electromagnetic coils areadapted to levitate the impeller, rotate the impeller, and center theimpeller laterally within the enclosure in response to receivedelectrical signals.

In accordance with one of the above-identified embodiments of theinvention, a “Synchronously Modulated Flow Compression Draw” (SMFCD) ALDsystem comprises a process space adapted to hold a substrate in fluidiccommunication with a single-impeller, low-volume GPS. The GPS isattached to a draw gas manifold that is operative to inject draw gasinto the GPS. Draw gas is injected into the GPS in such a manner as tocause the GPS to at least in part modulate the pressure and gas flowrate conditions in the process space in order to achieve highproductivity ALD. When sweep gas is injected into the process spaceduring sweep sub-steps, no draw gas is injected into the GPS and the GPSdisplays a moderate compression ratio. This allows the sweep gas toachieve relatively low pressures and relatively high gas flow rateswithin the process space. In contrast, while precursor gases areinjected into the process space during injection and reaction sub-steps,draw gas is concurrently injected into the GPS at such a draw gas flowrate that the pressure in the GPS is raised and its compression ratioreduced. This, in turn, allows precursor gases injected into the processspace to achieve relatively high pressures and relatively low gas flowrates within in the process space with minimal precursor waste.Downstream from the GPS, an abatement gas is injected into an abatementspace during sweep sub-steps to react with any un-reacted precursor gasthat is swept from the process space. The abatement gas reacts with theun-reacted precursor gas to form a film on an abatement surface.

Advantageously, aspects of the invention provide several advances to theart of ALD. Aspects of the invention, for example, may allow highproductivity ALD systems to be implemented with substantially largerprocess spaces and with less concern about pressure non-uniformities anddesign tolerances. Moreover, aspects of the invention may further allowALD systems to be implemented with higher sweep gas flow rates duringsweep sub-steps at reduced pumping requirements. They may also allow ALDsystems to be implemented with rapid and effective reactive precursorinjection into the process space and minimized precursor waste. Inaddition, aspects of the invention may allow ALD systems to beimplemented with higher overall process space pressures during reactionsub-steps resulting in faster deposition rates for a given substratetemperature and the ability to efficiently perform deposition at lowersubstrate temperatures. Finally, as one last example, aspects of theinvention may further allow ALD systems to be implemented with moreefficient abatement of precursor waste.

These and other features and advantages of the present invention willbecome apparent from the following detailed description which is to beread in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 shows a schematic diagram of an SMFD ALD system in accordancewith the prior art;

FIG. 2 shows a schematic diagram of an SMFCD ALD system in accordancewith an illustrative embodiment of the invention;

FIG. 3 shows a plan view of an illustrative single-impeller GPS for usein the FIG. 2 SMFCD ALD system;

FIG. 4 shows a sectional view of the FIG. 3 GPS;

FIG. 5 shows a perspective view of the impeller in the FIG. 3 GPS;

FIG. 6 shows a sectional view of an illustrative double-impeller GPS foruse in the FIG. 2 SMFCD ALD system;

FIG. 7 shows a sectional view of a wafer-processing SMFCD ALD system inaccordance with an illustrative embodiment of the invention; and

FIG. 8 shows a schematic diagram of the FIG. 2 SMFCD ALD system with theaddition of an abatement GPS.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described with reference to illustrativeembodiments. For this reason, numerous modifications can be made tothese embodiments and the results will still come within the scope ofthe invention. No limitations with respect to the specific embodimentsdescribed herein are intended or should be inferred.

As used herein, the symbol P_(x) ^(y) shall refer to the pressure atlocation x during the sub-step y, where y=s for sweep sub-steps, y=i forinjection sub-steps, and y=r for reaction sub-steps. Moreover, as usedherein, a GPS is meant to encompass any apparatus operative to pump gasby means of a moving surface.

FIG. 2 shows a schematic diagram of an illustrative SMFCD ALD system 200in accordance with an illustrative embodiment of the invention. TheSMFCD ALD system comprises a process gas manifold 201 operative tointroduce precursor gases and sweep gas into a process space 202containing a substrate 203. The process gas manifold comprises a firstprecursor gas source 204, a sweep gas source 205, and a second precursorgas source 206. The flows of these process gases into the process spaceare regulated by a first precursor gas valve 207, a sweep gas valve 208,and a second precursor gas valve 209, respectively. Further downstream,gas leaving the process space travels through a process space FRE 215into a GPS 216 through a GPS inlet 217. A draw gas manifold 220 allows adraw gas to be injected into the GPS. The draw gas manifold comprises adraw gas source 221 that is connected to a first draw gas valve 222 andan associated first draw gas FRE 223, as well as to a second draw gasvalve 225 and an associated second draw gas FRE 226. Gas leaving the GPStravels through a GPS outlet 232 and a GPS FRE 235 into an abatementspace 240, within which lies an abatement surface 242. An abatement gassource 250 is plumbed into the abatement space through an abatement gasvalve 251. The system is pumped by a vacuum pump 260.

Advantageously, some of the above-identified elements may be implementedusing commercially available parts, although they are combined in novelcombinations. The above described gas sources 204, 205, 206, 221, 250may, for example, comprise conventional gas containers (e.g., gascylinders) in combination with conventional regulators or conventionalelectronic pressure controllers. The above-described gas valves 207,208, 209, 222, 225, 251 may comprise, for example, conventional solenoidvalves, piezoelectric valves, electronic fuel injectors, proportionalvalves, or mass flow controllers (MFCs). Finally, the above-describedFREs 215, 223, 226, 232 may comprise adjustable components such asconventional metering valves, proportional valves, heated orifices, orMFCs. The FREs may also be implemented as passive components such asopenings and baffles designed into a vacuum chamber, capillaries, orporous materials.

As indicated in FIG. 2, the GPS 216 is in fluidic communication with theprocess space 202. This, in turn, allows the GPS to at least partiallyregulate the gas pressures and gas flow rates within the process space.As discovered by Gaede in 1912, the rotating blades of an impeller in aGaede pump can effectively serve as a pump throughout the molecular andthe transition flow regimes that are commonly used to characterize thetransport states of gases. The molecular and transition regions of a gasmay be defined by a Knudsen Number, K_(N)=D/λ, where λ is the mean freepath of the gas and D is a relevant geometrical factor (e.g., thespacing between blades of the impeller). Gaede pumps reach a maximum,substantially pressure-independent pumping speed within the molecularflow regime of K_(N)<1. In contrast, within the transition flow regime,defined by 1<K_(N)<110, the pumping speed falls from higher values atK_(N)≦1 down to near zero at K_(N)≈110 (where the gas enters its laminarviscous flow regime). Pumping speed and the related compression ratio(i.e., the ratio of the pressure at the pump outlet to the pressure atthe pump inlet) are determined by the rotation speed of the impellers,the specific design of the rotating and static elements of the pump, aswell as the molecular mass and temperature of the gas. Accordingly,compression ratios are easily adjusted by modifying these parameters ina manner that will be familiar to one skilled in the art.

Like the SMFD ALD system 100 shown in FIG. 1, the SMFCD ALD system 200in FIG. 2 is designed to deposit a material on a substrate by ALD usinga series of process cycles. Each process cycle comprises the sixinjection, reaction, and sweep sub-steps described earlier.Representative settings for the various valves during these sixsub-steps are preferably in accordance with the settings set forth inTable 1.

TABLE 1 Sub-step 207 208 209 222 225 251 Injection 1 Open Closed ClosedOpen Closed Closed Reaction 1 Periodic Closed Closed Open Closed ClosedSweep 1 Closed Open Closed Closed Closed Open Injection 2 Closed ClosedOpen Closed Open Closed Reaction 2 Closed Closed Periodic Closed OpenClosed Sweep 2 Closed Open Closed Closed Closed OpenAs indicated in this table, the draw gas manifold 220 does not injectany draw gas into the GPS 216 during the sweep sub-steps. Rather sweepgas from the sweep gas source 205 is flowed through the process space202 and the GPS 216 at a sweep gas flow rate, Q_(s), in order to rid theprocess space of remaining precursor gas from proceeding reactionsub-steps. This sweep gas flow rate results in P₂₃₂ ^(s)≈Q_(s)/C₂₃₅+P₂₄₀^(s) after the pressure is allowed to stabilize, where C₂₃₅ is theconductance of the GPS FRE 235. If the GPS is designed to provide acompression ratio, K_(GPS), of K_(GPS)=10 under these conditions, thenP₂₀₂ ^(s)≈P₂₃₂ ^(s)/10 after stabilizing, assuming that the processspace FRE 215 has a high enough conductance that P₂₀₂ ^(s)≈P₂₁₇ ^(s).Accordingly the sweep gas residence time in the process space, τ₂₀₂^(s), is:

$\begin{matrix}{\tau_{202}^{s} = \frac{V_{202} \times P_{232}^{s}}{10 \times Q_{s}}} & (3)\end{matrix}$

where V₂₀₂ is the free-space volume of process space.

In contrast, as further indicated in Table 1, during the injectionsub-steps, the process gas manifold 201 injects either the firstprecursor gas or the second precursor gas into the process space 202while the draw gas manifold 220 concurrently injects draw gas into theGPS 216. If, as is preferable, the gas flow rate of the draw gas,Q_(DG), is set such that Q_(DG)>Q_(s), P₂₃₂ ^(i) increases and theK_(GPS) decreases, resulting in P₂₀₂ ^(i)>P₂₀₂ ^(s) after P₂₀₂ ^(i) hashad a chance to stabilize. For example, if K_(GPS)≈2 and P₂₃₂^(i)≈1.5P₂₃₂ ^(s), then P₂₀₂ ^(i)=7.5P₂₀₂ ^(s) at the end of theinjection sub-step. During the injection sub-step, P₂₁₇ ^(i) ispreferably synchronized with the reactive precursor injection flow suchthat the increase in P₂₀₂ ^(i) predominantly corresponds to theinjection of precursor gas. Advantageously, a factor of about 7.5pressure increase in the process space represents ALD precursor loadingup to greater than about 86% with virtually no wasted chemical.

Following the injection sub-step, the appropriate precursor gas valve207, 209 is switched off while the draw gas manifold 220 maintainsQ_(DG). This acts to maintain the pressure of the precursor gas in theprocess space 202 that was achieved by the prior injection sub-step(i.e., P₂₀₂ ^(r)≈P₂₀₂ ^(i)). During the reaction sub-steps, additionalperiodic injections of precursor gas may be used to replenish depletedprecursor gas by sequencing the appropriate precursor gas valve 207, 209as necessary. Since P₂₀₂ ^(r)≈P₂₁₇ ^(r), the pressure drop across theprocess space FRE 215 is effectively about zero (i.e., ΔP_(Draw)≈0),causing little or no flow across this FRE and effectively isolating theprocess space 202 from the GPS 216. Any transient differences betweenP₂₁₇ ^(r) and P₂₀₂ ^(r) will result in a rapid self correction of P₂₀₂^(r) to match P₂₁₇ ^(r) following the shutoff of the precursor gasvalve.

As can be seen from the previous discussion, methods and apparatus inaccordance with aspects of this invention utilize K_(GPS) to modulatethe pressure and gas flow rate in the process space 202. Preferably, theGPS 216 is designed to maintain a compression ratio ˜10<K_(GPS)<˜50under the low-pressure/high-gas-flow-rate conditions best suited for thesweep sub-steps. High-pressure/low-gas-flow-rate conditions best suitedfor the injection and reaction sub-steps are triggered by a flow surgeof draw gas into the GPS. The flow surge quickly elevates P₂₃₂ ^(i)pushing the point where K_(N)≈110 and K_(GPS)=1 upstream in the GPS(i.e., the flow surge causes a least a portion of the gas in the GPS tobe in its laminar viscous flow regime where there is little or nocompression). As a result the total compression ratio modulates into alower level. The combination of higher P₂₃₂ ^(i) and a lower K_(GPS)leads to a higher P₂₀₂ ^(r). Because the draw gas manifold 220 utilizesthe first draw gas valve 222 and the first draw gas FRE 223 to injectdraw gas into the GPS during the first injection/reaction sub-steps, andbecause the draw gas manifold utilizes the second draw gas valve 225 andthe second draw gas FRE 226 to inject draw gas into the GPS duringsecond injection sub-steps, Q_(DG) can be made different for the firstand second injection/reaction sub-steps, if so desired. As a result,P₂₀₂ ^(r) may be more readily tailored to each precursor gas. Ifdesired, it is also contemplated that a different draw gas compositionmay be utilized during the first and second injection/reaction sub-stepsrather than using a single composition for both.

As indicated earlier, the time required to achieve saturation of a givenALD reaction sub-step is typically determined by the rate of precursorgas molecules impinging on the substrate 203. This rate of impingement(i.e., flux), φ, is proportional to the partial pressure of theprecursor gas in the process space 202. As a result, the ability toachieve higher P₂₀₂ ^(r) by injecting precursor gas into the processspace while concurrently injecting draw gas into the GPS 216 provides anopportunity to substantially speed up the reaction time. Real-worldexperience with prior-art SMFD ALD systems suggests that methods andapparatus in accordance with aspects of the present invention may beable to achieve process space pressures during reaction sub-steps ofabout 5 to about 10 times higher than those that can be achieved byprior art systems. This, in turn, represents the opportunity to speedupreaction times by that factor. For example, the pressure in the processspace may be raised from about 50 milli-Torr (mTorr) during sweepsub-steps up to about 375 mTorr during the reaction sub-steps using theSMFCD ALD system 200. At 86% concentration, the partial pressure of322.5 mTorr corresponds, for example, to a trimethylaluminum (TMA) (acommon first precursor gas) φ≈6.5×10¹⁹ molecules per square centimeterper second at 150° C. Under these conditions, the TMA reaction willsaturate within less than about 30 ms at 150° C. and less than about 8ms at 300° C. These reaction saturation times represent substantialimprovement to the art of ALD.

In addition, aspects of the invention may allow the implementation ofprocess spaces larger than those contemplated in the prior art, therebyallowing ALD to be performed on larger substrates. As indicated byequation (3) above, τ₂₀₂ ^(s) is reduced by 1/K_(GPS) when using a GPSin the manner described herein. As a result, a substantially largerprocess space volume, V₂₀₂, may be effectively swept for a given sweepsub-step time than would be the case without GPS compression.Furthermore, the high speed rotation of the GPS impeller acts tosubstantially average over any draw gas injection non-uniformities inthe GPS and any spatial non-uniformities in the process space FRE 215.These improved gas dynamics may act to reduce any undesirable spatialpressure gradients in the associated process space during injection andreaction sub-steps. This too provides the opportunity to implementlarger process spaces for which pressure non-uniformities may havepreviously been a concern.

Moreover, injection of draw gas into the GPS 216 has added advantageswith respect to abatement. The injection of draw gas into the GPSsubstantially isolates the process space 202 from the abatement space240. This allows the choice of abatement gas from abatement gas source250 to not affect the ALD process itself. Furthermore, because the GPSis compressing the gas that passes through it during the sweepsub-steps, P₂₃₂ ^(s) is substantially higher than it would be in asystem without such compression. This facilitates higher pressureabatement reactions without the risk of abatement gases back-streaminginto the GPS or into the process space.

As indicated earlier, a GPS with a moderate K_(GPS) in the range ofbetween about 10 and about 50 is preferable for SMFCD ALD applications.As can be appreciated by those who are skilled in the art, currentlyavailable Gaede pumping devices such as turbomolecular pumps andmolecular drag pumps may not be suitable for SMFCD ALD applications fora variety of reasons. In particular, these types of available devicestypically have a large volume and/or excessive compression ratio. In thecase of turbomolecular pumps, commercially available pumping devicestypically have multiple stages wherein rotors combine multiple impellersinto a single integral rigid unit. These impellers typically integrateinto a staggered arrangement of matching stators to facilitate highcompression. Accordingly, these devices have a very high compressionratio and are not adequate for SMFCD ALD and, similarly, are not readilysuitable for modifications and adaptation to an SMFCD ALD apparatus.Operating these commercial Gaede pumping devices at lower speed could beused to suppress their otherwise very large compression ratios, buttheir large volumes and reduced base conductance makes them slow torespond. Likewise, in the case of molecular drag pumps, low inletconductances are contradictory to optimized SMFCD ALD methods andapparatus.

Given the requirement for a rather moderate K_(GPS), GPS designsimplementing one or two high-speed rotating impellers may besatisfactory for SMFCD ALD applications, and, advantageously, may alsobe implemented within low-profile, low-volume modules. FIGS. 3 and 4show a plan view and a sectional view, respectively, of asingle-impeller GPS 216 for use the SMFCD ALD system 200. The GPScomprises a ring-shaped enclosure 302 that laterally surrounds animpeller 304, which is also shown in perspective view in FIG. 5. Theimpeller comprises an array of twisted blades 312, each blade emergingradially at a small angle, (e.g., about five degrees) from an inner hub314 and smoothly twisting to a larger angle (e.g., about 45 degrees) atthe perimeter-end where it is attached to an outer rim 316. Permanentmagnets 320 with alternating polarity orientations are embedded into theouter rim of the impeller, and a complementary set of electromagneticcoils 324 are disposed on the ring-shaped enclosure proximate to thesepermanent magnets and the outer rim. An array of position sensors 330 isalso evenly distributed on the outer rim of the impeller and theircorresponding position detectors 332 are placed in opposition on thering-shaped enclosure. Signals provided by the position sensors and theposition detectors are used to provide feedback for magnetic levitationand rotation control by a controller, as is known in the art.

It will be noted that the first draw gas valve 222 is also shown in thesectional view in FIG. 3. (The second draw gas valve 225 is not shownfor simplicity of presentation.) Once the first draw gas valve isopened, the draw gas is injected into a gap 340 in the GPS 216 betweenthe ring-shaped enclosure 302 and the outer rim 316 of the impeller 304.The impeller includes radially distributed feed holes 350 that act totransport at least some of the injected draw gas from the gap to theblades 312 of the impeller. Given the high-speed rotation of theimpeller (e.g., about 30,000 rotations per minute), the GPS can beexpected to substantially ameliorate any draw gas distributionimperfections due to, for example, manufacturing tolerances of thesefeed holes.

To rotate the impeller 304 in the GPS 216, alternating current is fed tothe electromagnetic coils 324 to effectively create a permanent magnetsynchronous motor (PMSM), which will be familiar to those skilled in theart. Moreover, in accordance with aspects of the invention, theelectromagnetic coils are also used to spatially center the impellerwithin the ring-shaped enclosure 302. This spatial positioning isperformed by purposefully creating magnetic force differences betweenpairs of electromagnetic coils located on opposing sides of thering-shaped enclosure. For example, when the position indicators 332indicate that the impeller has drifted laterally from center indirection 360 in FIG. 3, the magnitude of the current throughelectromagnetic coil 324A is made different from that in electromagneticcoil 324B so that the magnetic force created by each electromagneticcoil is not equal. These unequal magnetic forces act to deflect theimpeller back to its centered position within the ring-shaped enclosure.

The impeller 304 is preferably constructed out of light weight materialsuch as aluminum, titanium, or high strength plastics to reduce bothdriving and levitation power consumption. Conventional molding orcasting techniques are suitable for constructing the impellers withembedded permanent magnets 320. Advantageously, the GPS 216 does notrequire bearings and vacuum-compatible motors and/or can-assemblieswhich are commonly implemented for suspending and driving impellers inGaede pumps. These components may disadvantageously add volume andpotentially corroding components into the exhaust flow path.

For illustrative purposes, the ring-shaped enclosure 302 of the GPS 216has dimensions similar to that of a conventional two-inch long ISO160flange. Advantageously, forming the GPS in this manner acts to limit thesize of the free space within the GPS and also makes the GPS easy tointegrate with other standard-sized vacuum elements. A two-inch-longISO160-based GPS, for example, defines a free volume of only about oneliter. This small volume allows extremely fast pressure/gas-flow-ratetransitions within the GPS (e.g., <1 ms). On the other hand, forming theGPS is this manner may act to limit the pumping speed of the GPS. AnISO160 flange has an open area conductance of about 2,300 liters persecond (1/s). However, unlike an open flange, only the area between theblades 312 of the impeller 304 is effectively open in the GPS 216. As aresult, the GPS may only have a conductance of about 2,000 l/s.Nevertheless, even though limited by conductance to this pumping speedvalue, suitable low-pressure/high-gas-flow-rate conditions may still beobtained during sweep sub-steps. If the SMFCD ALD system 200 is run suchthat Q_(s)=8 standard liters per minute (sLm) (a very reasonable value),for example, P₂₁₇ ^(s)≈50 mTorr with a pumping speed of 2,000 l/s. Underthese conditions, a 50 liter free-volume process space 202 will have acharacteristic τ₂₀₂ ^(s)≈25 ms. This means that any un-reacted precursorgases in the process space will be adequately swept within only about125 ms (i.e., 5 τ₂₀₂ ^(s)).

If K_(GPS)=10 and P₂₁₇ ^(s)≈50 mTorr, P₂₃₂ ^(s)≈0.5 Torr. In such acase, the vacuum pump 260 in the SMFCD ALD system 200 may comprise, asjust one example, a moderately sized roots blower pump. One suchcommercially available roots blower pump is the Okta WKP2000 availablefrom Pfeiffer Vacuum (Asslar, Germany). With an ISO160 inlet, this pumpcan handle up to 22 sLm of gas flow at 0.5 Torr. Remarkably, themoderate K_(GPS) alleviates the need for extremely large pumps. Forexample, if K_(GPS)=1, Q_(s)=8 sLm, and P₂₃₂ ^(s)=50 mTorr, atremendously large roots blower pump such as the Okta WKP18000 and anISO400 pumping port might be required. On a comparative basis, the OktaWKP18000 has dimensions of about 2,630×1,460×1,000 millimeters (mm)length times width times height (“L×W×H”) while the Okta WKP2000 hasdimensions of only about 1,304×502×420 mm L×W×H.

The GPS 216 used in the SMFCD ALD system 200 may be configured withmultiple impellers. FIG. 6 shows a sectional view of a double-impellerGPS 216′ for use in the SMFCD ALD system. This GPS embodiment comprisesa ring-shaped enclosure 601 that laterally surrounds an upper impeller603 and a lower impeller 605. The upper impeller rotates in acounter-clockwise direction while the lower impeller rotates in aclockwise direction (i.e., the two impellers rotate in oppositedirections relative to one another). Draw gas is fed into a planetarymanifold 611 (i.e., a gas pathway that orbits the ring-shaped enclosure)and is further injected into a gap 613 between the impellers through aset of holes 615. To facilitate levitation and rotation, the upperimpeller is equipped with upper permanent magnets 617 and upper positionsensors 619, and is surrounded by upper electromagnetic coils 621 andupper position detectors 623. Likewise, the lower impeller is equippedwith second permanent magnets 625 and second position sensors 627, andis surrounded by second electromagnetic coils 629 and second positiondetectors 631. Note the insets showing the opposite blade designs of thetwo impellers, which accommodate their opposite directions of rotation.Opposite directions of rotation provide passive compression ratioenhancement as known in the art for Gaede pump designs. However, giventhe mild compression needed for the GPS, double-impeller designs withrigidly coupled impellers that rotate in the same direction may also beappropriate for SMFCD ALD. Rigidly coupled impellers may provide highercompression ratios if a stator with opposite blade directions is placedin the gap between the impellers. A double-impeller design may providebetter definition of draw gas spatial injection and a higher K_(GPS) atthe cost of additional complexity and possibly a somewhat extendedtransition time.

FIG. 7 shows an SMFCD ALD system 700 in accordance with an illustrativeembodiment of the invention that is suitable for depositing materials onsubstrates such as semiconductor wafers. The SMFCD ALD system comprisesa heated stage 702 adapted to hold a substrate 704, and a slit-valve 706that defines a process space 708. The slit-valve is operative to beraised and lowered in order to insert and remove the substrate, which isfurther facilitated by a lift-pin mechanism 710. The lift-pin mechanismis adapted to raise the substrate off of the heated stage duringsubstrate transfer operations. A ring-shaped impeller 712 forms the GPSfor the system and laterally surrounds and rotates around the heatedstage. To facilitate levitation and rotation, the ring-shaped impellercomprises an extended outer rim 714 in which permanent magnets 716 areembedded across from electromagnetic coils 718. The extended outer rimis useful to locate the permanent magnets below the congested areacomprising the slit-valve. Position sensors 720 and position detectors722 are implemented in manner similar to that described above. Draw gasis fed into the GPS through a planetary manifold 724 and is allowed topass to the blades of the impeller through radially distributed holes726 in an upper portion of the ring-shaped impeller's outer rim.Downstream from the ring-shaped impeller, an abatement space 728 isequipped with an abatement surface 730.

Finally, FIG. 8 shows a schematic diagram of an SMFCD ALD system 200′.This system is identical to the SMFCD system 200 in FIG. 2 except forthe addition of an abatement GPS 802 at the outlet of the abatementspace 240, as well as the addition of an abatement draw gas manifold804. As a result, for ease of understanding, elements that are theidentical in both the SMFCD ALD system 200′ and the SMFCD ALD system 200are labeled with identical reference numerals. In the SMFCD ALD system200′, the abatement draw gas manifold comprises an abatement draw gassource 806, an abatement draw gas valve 808, and an abatement draw gasFRE 810. The abatement draw gas manifold is preferably utilized totemporarily raise the pressure in the abatement space during an initialperiod of each sweep sub-step by injecting draw gas into the abatementGPS. This initial period may, for example, last between about τ_(s) andabout 2τ_(s) when the sweep sub-step has a total period of about 5τ_(s).These synchronized pressure-up modulations boost the abatement processefficiency in the abatement space during the initial periods of thesweep sub-steps in order to obtain improved handling of precursor wastefrom the process space 202. Experience indicates that the amount ofprecursor waste entering the abatement space is substantially greaterduring the initial periods of the sweep sub-steps than it is in thelatter portions of the sweep sub-steps.

It should again be emphasized that the above-described embodiments ofthe invention are intended to be illustrative only. Other embodimentscan use different types and arrangements of elements for implementingthe described functionality. For example, rather than implementing a GPSin the manners shown in FIGS. 3-7, a GPS could be implemented using aconventional impeller levitation and drive design wherein an impeller issupported by mechanical bearings and/or driven by a vacuum-compatiblemechanical motor. Alternatively, an SMFCD ALD system in accordance withaspects of the invention may be configured so that more than onecomposition of sweep gas, more than one composition of draw gas, and/ormore than one composition of abatement gas are available and utilized ina single ALD process cycle. These numerous alternative embodimentswithin the scope of the invention will be apparent to one skilled in theart.

In so much as aspects of the present invention teach methods ofmanufacture, the invention is further intended to encompass products ofmanufacture that are formed at least in part using these methods.Moreover, all the features disclosed herein may be replaced byalternative features serving the same, equivalent, or similar purpose,unless expressly stated otherwise. Thus, unless expressly statedotherwise, each features disclosed is one example only of a genericseries of equivalent or similar features.

What is claimed is:
 1. An apparatus for depositing a material on a substrate, the apparatus comprising: a process space adapted to hold the substrate; a Gaede pump stage (GPS) in fluidic communication with the process space; and one or more gas manifolds adapted to inject a precursor gas into the process space while injecting a draw gas at a draw gas flow rate directly into the GPS such that the injected precursor gas achieves a precursor pressure and a precursor gas flow rate in the process space, and to sweep substantially all of the precursor gas remaining in the process space from the process space by injecting a sweep gas into the process space such that the injected sweep gas achieves a sweep pressure and sweep gas flow rate in the process space; wherein the precursor gas flow rate is lower than the sweep gas flow rate.
 2. The apparatus of claim 1, wherein the precursor pressure is higher than the sweep pressure.
 3. The apparatus of claim 1, wherein the one or more gas manifolds are further adapted to inject a second precursor gas into the process space while injecting a second draw gas at a second draw gas flow rate directly into the GPS such that the injected second precursor gas achieves a second precursor pressure and a second precursor gas flow rate in the process space, the second precursor gas flow rate being lower than the sweep gas flow rate.
 4. The apparatus of claim 3, wherein the second precursor pressure is higher than the sweep pressure.
 5. The apparatus of claim 3, wherein the second draw gas has substantially the same composition as the draw gas.
 6. The apparatus of claim 3, wherein the second draw gas flow rate is substantially the same as the draw gas flow rate.
 7. The apparatus of claim 3, wherein the second precursor pressure differs from the precursor pressure.
 8. The apparatus of claim 1, wherein the draw gas flow rate is higher than the sweep gas flow rate.
 9. The apparatus of claim 1, wherein the GPS displays a lower compression ratio while the precursor gas is injected into the process space than it does while the sweep gas is injected into the process space.
 10. The apparatus of claim 1, wherein the GPS comprises only one impeller.
 11. The apparatus of claim 1, wherein the GPS comprises a plurality of impellers.
 12. The apparatus of claim 11, wherein two of the plurality of impellers rotate in opposite directions.
 13. The apparatus of claim 1, wherein the GPS comprises an impeller laterally surrounded by a ring-shaped enclosure.
 14. The apparatus of claim 13, wherein at least one of the one more gas manifolds is adapted to inject the draw gas into a gap between the impeller and the ring-shaped enclosure.
 15. The apparatus of claim 1, wherein the GPS comprises an impeller that includes a plurality of blades that span radially from an inner hub to an outer rim.
 16. The apparatus of claim 15, wherein a plurality of magnets are distributed along the outer rim of the impeller.
 17. The apparatus of claim 15, wherein the impeller may be made to rotate by providing electrical signals to a plurality of electromagnetic coils distributed proximate to the outer rim of the impeller.
 18. The apparatus of claim 17, wherein the impeller may be further made to shift laterally by providing electrical signals to the plurality of electromagnetic coils.
 19. The apparatus of claim 15, wherein the impeller comprises a plurality of holes in its outer rim.
 20. The apparatus or claim 1, wherein the GPS comprises a ring-shaped impeller, the ring-shaped impeller laterally surrounding and adapted to rotate around one or more other elements of the apparatus.
 21. The apparatus of claim 20, wherein the ring-shaped impeller laterally surrounds and is adapted to rotate around a stage adapted to hold the substrate.
 22. The apparatus of claim 1, further comprising an abatement space in fluidic communication with the GPS, the abatement space comprising an abatement surface on which the precursor gas may be converted to a film on the abatement surface in the presence of an abatement gas.
 23. The apparatus of claim 22, further comprising: an abatement GPS in fluidic communication with the abatement space; and an abatement draw gas manifold adapted to inject an abatement draw gas into the abatement GPS.
 24. The apparatus of claim 1, further comprising a roots blower pump.
 25. A Gaede pump stage (GPS) comprising: an impeller comprising a plurality of blades that span radially from an inner hub to an outer rim; an enclosure laterally surrounding the impeller; a plurality of permanent magnets attached to the outer rim of the impeller; and a plurality of electromagnetic coils attached to the enclosure and disposed proximate to the outer rim of the impeller, the plurality of electromagnetic coils adapted to levitate the impeller, rotate the impeller, and center the impeller laterally within the enclosure in response to received electrical signals.
 26. The GPS of claim 25, wherein the GPS comprises only a single impeller.
 27. The GPS of claim 25, wherein the GPS comprises a plurality of impellers.
 28. The GPS of claim 27, wherein two of the plurality of impellers rotate in opposite directions.
 29. The GPS of claim 25, wherein the enclosure is ring-shaped.
 30. The GPS of claim 25, wherein the enclosure defines a volume of less than about one liter.
 31. The GPS of claim 25, wherein the impeller comprises a plurality of holes in its outer rim.
 32. The GPS or claim 25, wherein the GPS comprises a ring-shaped impeller, the ring-shaped impeller laterally surrounding and adapted to rotate around one or more other elements.
 33. The GPS of claim 32, wherein the ring-shaped impeller laterally surrounds and is adapted to rotate around a stage adapted to hold a substrate. 